1. Field of the Invention
The present invention relates to a radiation system, a lithographic apparatus including a radiation system, and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic apparatus may include a radiation generator arranged to provide a radiation beam. The radiation generator is positioned relative to an illumination system that is arranged to process the beam and provide a projection beam. When the radiation generator is not correctly positioned, the radiation beam will enter the illumination system in an erroneous way. A zero sensor may be used arranged to measure the relative position of the radiation generator in 6 degrees of freedom (DoF). A zero sensor includes a laser and a corner cube that reflects the laser beam on to a detector. This type of measuring system provides accurate alignment if the laser and corner cube can be positioned close together and a small measurement range is needed. However, to align a radiation generator relative to an illumination system, such a zero sensor has either a measurement range or a measurement accuracy that does not meet the requirements.
In current DUV (deep ultra violet) and VUV (very ultra violet) systems the alignment of the position of a radiation generator with reference to an illumination system is done by measuring the position and/or tilt of the beam coming from the radiation generator with respect to the illumination system. In such radiation systems, a part of the radiation beam is directed to a beam measuring system by way of a mirror with a reflection coefficient of, for example 4%, and a transmission coefficient of, for example 95%, the residual intensity is lost due to absorption. The reflected part is processed by an optical system including lenses to project a focus spot or a demagnified image on a position sensor, such as CCD sensors, CMos, Quad Cells and PSD, referred to herein as “position sensors”. Each position sensor is arranged to measure the near field or far field of the beam in two degrees of freedom. By using several position sensors, the position and tilt of the radiation beam can be determined in four degrees of freedom. Using a partly reflective mirror will result in a loss of radiation (i.e. energy) in the direction of the rest of the illumination system. Furthermore, such a partly reflecting mirror is not suitable for very short wavelengths in combination with the high radiation intensity used in EUV lithography systems.